Cylindrical portions of fluid pressure cylinders such as hydraulic cylinders, air cylinders or the like are required to have roundness of 25 μm for hydraulic cylinders and 150 μm for air cylinders. For example, because aluminum extrusion hollow shapes used for these cylinders have inadequate precision by normal extrusion only, roundness or dimensional precision of cylindrical portions of most products thereof are improved by machining such as cutting or by post-machining burnishing. However, a material, the cylindrical portion of which has a small bore or a longitudinal size larger than the bore, can not be machined. Therefore, the current situation has been such that, for example, yield is declining for not achieving required roundness, or productivity is staying low for having to machine the pieces, one by one, of a long sized material which must be cut into product length beforehand.
On the other hand, electrolytic integrated polishing technology, a hybrid of elution by electrolyte and abrasion by abrasives, is known as a method for polishing the surface of metal with a high precision. The electrolytic integrated polishing technology, applied to mirror finishing of the internal surface of metallic tubes, generally inserts a rotation shaft, to the top of which a tool electrode is attached, into a metallic tube, rotates the metallic tube about the rotation shaft, then supplies electrolyte inside the metallic tube and passes current, and polishes the internal surface of the metallic tube by a grindstone attached to the tool electrode, withdrawing the rotation shaft slowly.
With such electrolytic integrated polishing methods for the internal surface of metallic tubes, abrasion of the grindstone prevents high precision mirror finishing due to inappropriate polishing of the internal surface. Therefore, apparatuses coping with abrasion of the grindstone by applying pressing force onto the grindstone via a leaf, a diaphragm or the like are conventionally known. (See Japanese Utility Model Laid-Open Publication No. Hei 4-130120, 5-86429 and so on.)
The above mentioned conventional method using a leaf can not preserve the desired polishing performance because pressing force decreases as the grindstone abrades, especially in narrow places such as the internal surface of a long sized metallic tube. Besides, there have been shortcomings such as the necessity of a specific insertion guide for inserting the polishing tool into the metallic tube because the leaf generates force by being bent to a predetermined extent.
On the other hand, although a method using a diaphragm can resolve the above mentioned problem caused by the leaf, a plurality of diaphragms corresponding to a plurality of grindstones must be individually disposed, which makes processing of the installation sections, and installation work complicated. Besides pressing force applied to individual grindstones is not uniform due to variation occurred in the plurality of diaphragms, which makes mirror finishing with a high precision impossible.
The above mentioned method is mainly applied to processing of the internal surface of pipes made from steel or stainless steel. Most apparatuses are horizontal types that can easily perform processing of long sized products, while there are few vertical types. (See Japanese Patent Laid-Open Publication No. Hei 3-98758.) Furthermore, there is no concrete example of application to aluminum extrusion shapes having a variety of external shapes.
Machining (cutting, for example) of the internal surface of the cylindrical portion of an aluminum extrusion hollow shape is more difficult than processing of external surface. Besides, long materials, with difficulties in processing due to problems with rigidity of the tool, must be cut short for processing in order to preserve a certain processing precision. Therefore, other than increase in cost, ultra high precision mirror finishing of aluminum alloy by cutting has been difficult because aluminum alloy, having a low rigidity which is about one-third of steel and a coefficient of thermal expansion twice as large as steel, is easy to be deformed by cutting resistance or cutting heat.
On the other hand, electrolytic integrated polishing which is an ultra high precision finishing technology with a roundness of finished surface equal to or smaller than 10 μm and a surface roughness equal to or smaller than 1 μm, can be applied to long sized pipes. However, a problem remains in that directly applying electrolytic integrated polishing methods and apparatuses that have been mainly used for polishing of the internal surface of steel or stainless steel tubes to polishing of the internal surface of a cylindrical portion of an aluminum extrusion hollow shape will not result in processing with a high precision.
For example, the inventors of the present invention failed to obtain the desired processing precision by electrolytic integrated polishing of the internal surface of an aluminum extrusion tube using a conventional electrolytic integrated polishing apparatus (a type in which axial directions of a long sized metallic tube and a tool electrode are horizontally oriented, and the metallic tube and the tool electrode are rotated in opposite directions with each other.) This is because processing needed be performed with the pressing force of the grindstone controlled at a low pressure due to softness and easiness to deformation of aluminum, having a low strength and a low rigidity which are about one-third of steel or stainless steel, and because precision of processing degrades by deflection of the axial center due to influence of weight of the tool electrode supported horizontally by the rotation axis.
Besides, since rotating metallic shapes having a variety of external shapes is virtually impossible, long sized cylindrical workpieces to which the above mentioned electrolytic integrated polishing apparatus can be applied are limited to cylinder pipes.
It is an object of the present invention, having been made considering the above mentioned problems with prior arts, to provide an electrolytic integrated polishing method and apparatus which enables high precision polishing of the internal surfaces of the cylindrical portions of metallic shapes having a variety of external shapes such as aluminum extrusion hollow shapes or the like, and to provide long sized cylindrical workpieces such as aluminum extrusion hollow shapes having internal surfaces of the cylindrical portions polished with a high precision.